1. Field of the Invention
The present invention relates to a liquid crystal display apparatus which can be connected to a computer in place of a cathode ray tube display apparatus. More particularly, the present invention relates to a liquid crystal display apparatus having a function to automatically adjust a frequency of a dot clock signal reproduced on the side of the liquid crystal display apparatus to an optimal value to fit to the frequency of a dot clock signal in a cathode ray tube display apparatus.
2. Description of the Related Art
Generally, a horizontal sync signal, a vertical sync signal and analog image signals R, G, B are inputted from a computer or a workstation to a display apparatus. The frequency of a dot clock signal in the computer and the workstation as an image signal source depends on the model and resolution of the image signal source. The relation of the dot clock signal frequency and the mode and resolution is shown in the following table 1.
TABLE 1 ______________________________________ horizontal vertical dot clock frequency frequency frequency model resolution (KHz) (Hz) (MHz) ______________________________________ A 600 .times. 400 24.827 56.424 21.053 B 640 .times. 480 31.469 70.087 28.322 C 832 .times. 624 31.469 74.000 57.283 ______________________________________
FIG. 1 is a block diagram illustrating the structure of a conventional liquid crystal display apparatus to which a computer is connected. In the conventional liquid crystal display apparatus, a microcomputer 35 measures a horizontal sync signal and a vertical sync signal during a predetermined time period and calculates the frequency of the horizontal sync signal and the frequency of the vertical sync signal based on the measuring result. Then, the microcomputer 35 estimates the model of the computer from the calculating result using a table 36 like the table 1. Consequently, the microcomputer 35 controls a phase locked loop (PLL) circuit 34 based on a dot clock value for the estimated computer such that a dot clock signal corresponding to the computer is reproduced. An A/D converter 31 samples an image signal in accordance with the dot clock signal reproduced by the PLL circuit 34 and converts into a digital signal. A data processing circuit 32 adjusts timing and so on such that the image signal converted into the digital signal can be displayed on a liquid crystal display unit 33.
However, there is a case where the reproduced dot clock signal is subtly different from the dot clock signal used in a computer, depending on the model and resolution of the computer. In such a case, a user conventionally adjusts the frequency of the dot clock signal using switches and so on such that blur, color deviation, fluctuation and so on can be eliminated, while confirming a display by the eyes.
There is known a liquid crystal display apparatus disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-160222) in which the above adjustment is automated. In this apparatus, a specific image signal for the adjustment is inputted from a computer. While the frequency of the dot clock signal is changed, whether or not an image data is correctly A/D-converted is determined in units of dots during one horizontal period. When all the A/D-converted data become correct, the correct dot clock state is established.
As mentioned above, in the conventional liquid crystal display apparatuses, when the image signal sources such as the computer and the liquid crystal display apparatus are subtly different from each other in the frequency of the dot clock signal, the frequency of the dot clock signal on the side of the liquid crystal display apparatus has been manually adjusted such that the blur and fluctuation can be eliminated.
Also, in the liquid crystal display apparatus disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-160222), the automatic adjustment is accomplished. In this case, however, it is required that the computer outputs the specific image signal for the adjustment.